MAC: Fix setting loader tunables

[zlei]

Summary

For a variable string, the len parameter should be set to the size of it
rather than 0. The latter means a constant null-terminated string.

Fixes: 3da1cf1e88f8 Extend the meaning of the CTLFLAG_TUN flag ...
Fixes: af3b2549c4ba Pull in r267961 and r267973 again ...
MFC after: 1 week

[olce]

Before the changes, setting security.mac.{lomac,biba}.trusted_interfaces would have no effect at all (because the trusted_interfaces[] arrays would be filled with 0 at load). I think this problem was introduced by the second fixed commit you listed, which means this has not worked for ~10 years at least.

[zlei]

CC @jhb .
Those two MAC modules were obtained from TrustedBSD Project. I see @jhb was back porting commits from FreeBSD to TrustedBSD. So I guess @jhb may be interested with this fix.

[jhb]

Hmm, I don't think TrustedBSD is an active project anymore and I have never committed to it. Perhaps you are thinking of CheriBSD?

[zlei]

In D48898#1115602, @jhb wrote:

Hmm, I don't think TrustedBSD is an active project anymore and I have never committed to it.

Sorry for the noise.

Perhaps you are thinking of CheriBSD?

Yes. I misremembered.